1. Field of the Invention
The present invention relates to audio speaker systems, and, more particularly, to audio speaker systems including acoustic transformers that transform wavefronts of one shape from primary waveguides into another shape for input into sound disseminating secondary waveguides.
2. Description of the Related Art
Typically, a horn-type loudspeaker consists of a driver coupled to an initial throat section. The geometry of the sound-radiating diaphragm of the loudspeaker driver may be a cone, a spherical dome, a flat piston, or an annular ring-radiating diaphragm.
It is well known that the angle of sound radiation of the loudspeaker driver is dependent on the dimensions of the radiating exit relative to the wavelength of sound that is being generated. When the wavelength of sound is large compared to the dimension of the driver exit, the resulting radiation pattern has a wide angle. When the wavelength of sound is small compared to the dimension of the driver exit, the resulting radiation pattern has a narrow angle.
The walls of a horn can only confine the radiation pattern; the walls cannot widen the pattern. If the pattern of sound radiated from driver is wider than the angle of the horn walls, then the sound from the driver will fill the horn and the horn walls will determine the resulting radiation pattern of the horn/driver combination.
On the other hand, if the pattern of sound radiated from driver is narrower that the horn walls, then the sound from the driver will radiate as a narrow beam through the horn and the resulting radiation pattern of the horn/driver combination will be substantially unaffected by the horn walls. In this latter case, where the angle of radiation from the driver exit is narrower than the desired coverage, several techniques have been used in the prior art.
One technique in the prior art to widen the angle of radiation of the driver exit is to pass the sound from the driver exit through an acoustic-transformer/geometry-transition that changes the shape from a round to a rectangular slot, wherein one dimension of the slot is smaller than that of the driver exit. If the smallest dimension of the rectangular slot is smaller than the wavelength of sound, then the radiation angle from the slot will be wide and the horn walls can control the angle of radiation from the horn/driver combination (see U.S. Pat. Nos. 4,187,926 and 4,308,932).
The transformation from round to rectangular can solve the problem in the direction where the slot is smaller than the driver exit. However, problems may still exist in the direction where the direction where the rectangular slot dimension is larger than the driver exit.
Another technique used in the prior art in addition to the rectangular slot is to apply vanes in the throat that spread out the acoustic energy, widening the radiation angle (see U.S. Pat. No. 4,685,532). The vanes are a brute force approach to spreading the pattern out.
In the former case, where the angle of radiation from the driver exit is wider than the desired coverage, the horn walls can control the angle of radiation from the horn/driver combination. However, for very narrow horn/driver radiation angles, the horn can become long enough to create practical problems. Several techniques have been used in the prior art to narrow the coverage angle in a shorter distance. These effectively use an acoustic-transformer/geometry-transition that transforms from the round driver exit to a rectangular slot wherein the wave front has been tailored to be substantially flat, resulting in a narrow radiation pattern. This may be substituted for the first part of the horn, shortening the overall length. These inventions use path way geometries to delay the arrival of the sound at the center of the rectangular slot, making the wave front at the rectangular slot substantially flat (see U.S. Pat. Nos. 5,163,167, 6,581,719 and 6,668,969).
The above describes horn/driver combinations with symmetric radiation angles. However, a horn may be designed to radiate sound energy asymmetrically, directing more energy out the top of the horn and less energy out the bottom. One technique in the prior art to achieve that is to pass the sound from the driver exit through an acoustic-transformer/geometry-transition that changes the shape from round to a tall slot with a semi-trapezoidal shape that is wider at the top than at the bottom. This geometric transition directs more energy towards the top. The trapezoidal-shaped slot is coupled to horn flares to define the radiation angles of the horn/driver combination. (see U.S. Pat. No. 5,020,630).
For substantially curved and substantially flat wavefronts, the prior art addresses the two extremes as independent devices—devices that are applicable for making the radiation pattern from the loudspeaker driver exit much wider, or devices for making the pattern much narrower. The prior art addresses asymmetrical energy distribution with slots of varying widths.
The propagation of sound in a horn may be described by the one-dimensional horn equation:
                              ∂          2                ⁢        ∅                    ∂                  r          2                      -                  c        2            ⁢                                    ∂            ∅                    ∂                                      ∂            x                    ⁢                      ∂            x                              ⁢              (                  log          ⁢                                          ⁢          S                )              -                  c        2            ⁢                                    ∂            2                    ⁢          ∅                          ∂                      x            2                                =  0where the scalar velocity potential, Φ, is described along the x direction, and the cross sectional area of the horn is given by S. The speed of sound c (e.g., the speed of pressure waves) may be defined by:c2=B/ρwhere B is the bulk modulus of a gas (such as air), and ρ is the fluid density of the gas. The acoustic impedance at the throat of a waveguide is determined by the size and shape of the input and output of the device, the expansion function S and the waveguide length. This is a one-dimensional approximation for determining the radiation impedance of an acoustic waveguide. So, for two acoustic paths to have equal impedance they must share the same input and output shape, length, and expansion function.
According to the prior art, when designing waveguides for the purpose of transforming the apparent shape of the source, certain assumptions are made regarding the nature of the source. U.S. Pat. No. 5,163,167, for example, assumes a planar circular isophase wave surface as the excitation for such a waveguide. The term “isophase” means that the sound wave produced would be similar to the sound wave produced by a single piston-like vibrating disk. It can be shown for all electromechanical transducers that there exists a high frequency limit where diaphragm mode shapes and/or acoustic effects produce a non-planar, non-isophase wave front.
What is neither disclosed nor suggested in the art is an acoustic waveguide that does not have the problems and limitations of prior art waveguides as described above.